The present invention relates to a rotational speed sensor for detecting the rotational speed of a rotating member such as a wheel for a vehicle.
Various rotational speed sensors have been conventionally proposed. For example, Japanese Laid Open Patent Publication No. 08-211081, published on Aug. 20, 1996 and Japanese Laid Open Patent Publication No. 08-327646, published on Dec. 13, 1996, disclosed rotational speed sensors having ring-shaped yokes, ring-shaped magnets and ring-shaped detection coils. Further, Japanese Laid Open Patent Publication No.08-094656, published on Apr. 4, 1996, and Japanese Laid Open Patent Publication No. 08-101223, published on Apr. 16, 1996, disclosed rotational speed sensors having rod-shaped yokes, rod-shaped magnets and rod-shaped detection coils. These rotational speed sensors include pulsar rings that are mounted on rotating members. When the pulsar rings are rotated, the pulsar rings cause a change in magnetic flux that are converted to electrical signals by the detection coils.
However, these conventional speed sensors are easily affected by external alternating magnetic fields that may be created by an underground power transmission line or a heating wire buried under the road. In other words, these conventional speed sensors may not detect rotational speed accurately in the presence of such external alternating magnetic fields.
To reduce these magnetic fields or induction noise, magnetic shields may be added to the speed sensors but such magnetic shields are often not perfect. The number of turns may be reduced for the detection coils to reduce such induction noise but output signals will become weaker. In short, induction noise may be somewhat reduced by conventional schemes but may not be reduced satisfactorily.